The present invention relates to a thermo-pressure fixing type printer, which forms images on a continuous form recording sheet and fixes the images thereon using a heat roller and a pressing roller.
Conventionally, an electrophotographic imaging apparatus which forms images in accordance with a so-called electrophotographic imaging method. In such imaging apparatus, a photoconductive member on the surface of a photoconductive drum is exposed to light to form a latent image thereon, which is developed by applying toner to form a toner image. Then, the developed toner image is transferred onto a recording medium such as a recording sheet or recording paper, and fixed thereon using a fixing unit.
As an example of such an imaging apparatus, a printer that prints images sequentially onto each segment (i.e., page) of a continuous form recording sheet (hereinafter referred to as a continuous recording sheet) has been known. Generally, such a printer is configured to feed the continuous recording sheet by a feeding unit such as a tractor unit that drives a tractor belt. Next to the feeding unit, a pair of fixing rollers (i.e., heat roller and pressure roller) are provided to apply heat and pressure to the continuous recording sheet bearing the images and passing therebetween.
Typically, the heat roller of the pair of fixing rollers is driven to rotate at a constant speed so that the continuous sheet nipped between the heat roller and the pressure roller is fed toward the direction similar to the feeding direction of the feeding unit.
In such a printer, if the continuous sheet is nipped between the heat roller and the pressure roller when feeding of the continuous sheet is terminated, the continuous sheet may be scorched as the continuous sheet is overheated. In order to prevent the continuous sheet from scorching, in a conventional printer, the heat roller and the pressure roller are spaced apart when the feeding of the continuous sheet is stopped so that the same portion of the continuous sheet is not held in the nip between the heat roller and the pressure roller. When the feeding is restarted, feeding by the feeding unit and the rotation of the heat roller are started firstly, and then, the heat roller and the pressure roller are brought into an operable condition (i.e., brought into contact) so that the continuous sheet is nipped therebetween and fed thereby.
Some printers are configured to discharge all the pages bearing images before the sheet feeding is stopped so that users can check all the printed images and/or cut off all the pages bearing the printed images. After the printed images are checked or the printed pages are cut off, and the following printing job is to be restarted, the top page of the continuous sheet bearing no image and remaining in the printer is pulled back to the transfer position so that the image is formed from the top page of the unprinted sheet, thereby no blank (non-printed) page being generated.
Generally, when the continuous sheet is heated by the pair of fixing roller, especially in a high temperature and high humidity atmosphere, the continuous sheet shrinks at the heated portion due to the evaporation of the moisture in the continuous sheet.
If the positional relationship between the heat roller and the pressure roller is constant, the heat and pressure applied to the continuous sheet is evenly distributed in the width direction. Thus, the continuous sheet shrinks substantially uniformly across the width direction. In such a case, no crinkles may be formed on the continuous sheet. However, if the heat roller and the pressure roller are moved to apart from each other, distribution of the heat and the pressure applied to the continuous sheet may be changed in the width direction as the heat roller and the pressure roller move. In this case, crinkles may likely be formed in the continuous sheet.
In the above-described type printer, i.e., the printer in which the leading edge of the non-printed page of the continuous sheet is fed back when the printing is restarted, the heat is applied to the continuous sheet as follows. When a printing job is finished, the rollers are moved apart relative to each other. At this stage, a perforated portion between a trailing edge of the printed page and a leading edge of the non-printed page is heated. Due to the unevenly distributed heat applied to the continuous recording sheet, crinkles are formed at the leading edge portion of the non-printed page. It should be noted that a perforation line typically includes a plurality of incisions evenly aligned in the width direction, and therefore, occurrence of the crinkles are significant.
If another printing job is restarted with this condition, the continuous sheet is fed back so that the leading edge of the non-printed page is located on an upstream side of the photoconductive drum, and then, fed forward so that the electrophotographic imaging process is performed. Since the crinkles are formed on the leading edge portion of the non-printed page as described above, valley portions of the crinkles may not contact the photoconductive drum, and therefore, the toner image may not be transferred onto the valley portions.
It is therefore an object of the invention to provide an improved electrophotographic printer in which the trailing edge of the printed page is discharged from the printer when a printing operation is terminated, and the continuous sheet is retracted so that the leading edge of the non-printed page is located on the upstream side of the photoconductive drum when the printing operation is restarted, the above-described deficiency is prevented.
For the above object, according to the invention, there is provided an imaging apparatus for forming an image on a continuous recording sheet, in accordance with an electrophotographic imaging process, and fixing the image on the continuous recording sheet by applying heat and pressure using a pair of fixing rollers including a heat roller and a pressure roller. The imaging apparatus is configured such that, when a printing operation is terminated, a trailing end of a printed portion of the continuous recording sheet is passed through the fixing unit and discharged from the printer. When another imaging operation is to be executed, the continuous recording sheet is reversely fed until a leading end of a non-printed portion of the continuous recording sheet is located at a predetermined upstream position with respect to a transfer position so that image formation can be executed from the non-printed portion. In such an imaging apparatus, when a printing operation is finished, and the printed portion of the recording sheet is discharged from the printer, the heat roller is moved from an operable position, where it is press-contacted with a pressure roller, to a retracted position, where it is spaced from the pressure roller by a predetermined amount. It should be noted that, when the heat roller is moved toward the retracted position, it is once stopped at an intermediate position between the operable position and the retracted position for a predetermined period of time, and then, moved further to the retracted position.
According to the above configuration, since the retracting movement of the heat rollers Is divided into a plurality of movements, the crinkles will not be formed on the leading edge portion of the non-printed page when fixing rollers are brought into the retracted state.
Optionally, the at least one intermediate position may include a position at which the heat roller and the pressure roller contact the recording sheet at a position close to the trailing end of the printed portion.
Further optionally, the roller driving mechanism may include a pair of arm members swingable about an axis. The one of the heat roller and pressure roller is supported at an end portion of the pair of arms, the one of said heat roller and pressure roller being swung as the pair of arm members swing about the axis.
In a particular case, the roller driving mechanism may include a cam mechanism which may include a rotating cam which is rotatable about a rotation axis, and a cam rotating system that rotates said rotating cam at a predetermined constant speed. The rotating cam is formed to have an arc-shaped cam portion, which is centered about the rotation axis.
Further optionally, the roller driving mechanism may include a cam mechanism, which is provided with a cam member formed with a cam profile, a driving system that drives said cam member to move, a cam position detecting system that detects a position of said cam member, and a control system that controls movement of said cam member. The control system may control the driving system to stop driving the cam member for a predetermined period of time when the cam position detecting system detects that the cam member is located at a predetermined position.
Still optionally, the imaging apparatus may perform image formation on a page basis, the page being defined as a segment on the recording sheet divided by perforation lines formed thereon at predetermined intervals.